The invention pertains to temperature sensing electrical circuits and devices for use with automatic toaster appliances and the like. A great number of different temperature sensing and timing schemes have been developed for providing an automatic toasting operation. Of these, there are only a few which combine an economical construction with a reasonably satisfactory operation, and none that are known to combine these characteristics in a truly optimum manner.
The simplest timing mechanism is one that works entirely on a timing principle wherein the toast cycle is tied to a pre-set time interval that may be adjusted for different toast colors. Such mechanism will perform in a consistent fashion to provide a given toast color when all starting conditions are the same, such as toast compartment temperature, bread loading and bread condition. If any one condition is different, results may be different. For example, for the same toast color setting and bread condition, a second toasting operation begun before the toast compartment cools down will result in a darker piece of toast than in the initial operation because of the different initial toast compartment temperatures. Correspondingly, at this toast color setting a frozen slice of bread will be much lighter or perhaps not toasted at all. This is because the applied heat is used primarily to bring the temperature to ambient and remove moisture from the bread.
Perhaps the most commonly employed sensing mechanism used today is a bimetallic timer and switch, the sensor element of which is a bimetallic strip having its own heater wire in association with it. The bimetal strip is remotely located from the bread so as to respond principally to the heater wire and to the ambient. At the initiation of a toasting cycle the heater is turned on to heat the bimetal strip to a peak temperature set in accordance with a desired toast color, whereupon the heater is turned off and the bimetal strip cools to a second temperature which causes a switch to operate and end the toasting cycle. This form of mechanism acts to compensate for different toaster compartment initial temperatures and therefore consistent results may be produced for successive toasting operations. However, the bimetallic timer will not compensate for variations in bread loading or bread condition. Thus, with the operation set for a given toast color setting, two slices of bread will tend to toast to a lighter color than for a single slice of bread, and three slices will tend to toast lighter still. Similarly, if the setting is proper for dry bread, moist bread will toast to a relatively light color.
A third mechanism employed in contemporary toaster appliances is a bimetallic thermal sensor and switch which includes a bimetallic strip sensor element placed in proximity with the bread for sensing the temperature of the bread and moving a lever arm to actuate the switch. Upon reaching a pre-set temperature that corresponds to a desired toast color, the bimetallic strip actuates its switch for ending the toasting cycle. The mechanism includes compensating means, which can be a separate bimetallic strip or an integral part of the primary bimetallic strip, to compensate for the spurious effects of ambient heating. Although this form of mechanism is capable of providing a uniform toast color under all starting conditions, because of the very exacting mechanical tolerances required in the construction and assembly of the mechanism, including an accurate spacing between the lever arm and the switch contacts and between the bimetallic strip and the bread surface, and the need to maintain these dimensions during continued use, it is difficult to avoid inaccuracies in the toasting operation.